Larynx

The larynx is a special part of the body that functions as an airway
to the lungs as well as providing us with a way of communicating (vocalizing).
These functions are all possible because of the skeletal components and
the muscles that act on them. Before learning the details, memorize the
various parts of the skeleton so that you can then visually place the muscles
in the correct places and appreciate how they do their jobs.

Skeleton of the Larynx

The skeleton of the larynx is made up of the hyoid bone and several
cartilages.

The thyroid cartilage is made up of two laminae that fuse
anteriorly for form the laryngeal prominence (Adam's apple). The angle
that they make is usually more acute in males and therefore, is more prominent.
The inferior horns articulate with the sides of the cricoid cartilage
and form the cricothyroid joint where the thyroid cartilage rocks back
and forth at this point.

The cricoid cartilage is the only complete cartilage of
the larynx. Anteriorly is the cricoid arch. The arch expands as you trace
it posteriorly where it forms a square-shaped lamina.

The arytenoid cartilages sit on top of the cricoid lamina,
posteriorly and articulate there at the cricoarytenoid joints. The arytenoid
cartilages slide medially and laterally, anteriorly and posteriorly and
rotate at these joints. The cartilage is pyramidal in shape with the base
being triangular in shape with 3 processes. The vocal process extends anteriorly,
the muscular process lies laterally and third process is not well defined.
The vocal ligament (cord) extends from the vocal process to the back side
of the thyroid cartilage. You can appreciate that any movement of the arytenoid
cartilage will have an effect on the placement of the vocal cords (making
them loose or taut, bring them together or spreading them apart).

The epiglottis is attached inferiorly to the thyroid cartilage
by a small stem. Its lateral and superior borders are free. The superior
border can be seen through the oral cavity.

Anterolateral View

Posterior View

Various parts of the larynx area closed by connective tissue membranes.
The thyrohyoid membrane was seen in the study of the neck
and is pierced by the internal laryngeal nerve and superior laryngeal artery.
It extends from the upper border of the thyroid cartilage to the greater
wing of the hyoid bone.
The quadrangular membrane is free at the top and bottom
but attached posteriorly to the arytenoid cartilage and anteriorly to the
side of the epiglottis. The lower free margin forms the false vocal cord
(or vestibular fold).
The cricothyroid membrane (or conus elasticus) extends
from the upper margin of the cricoid cartilage to attach to the back of
the thyroid cartilage anteriorly and the arytenoid cartilage posteriorly.
Its upper free margin is the vocal ligament (true vocal cord).

Muscles of the Larynx

The muscles of the larynx can be difficult to clean and identify. They
consists of muscles that change the opening of the glottis as well as the
tenseness of the vocal cord, thereby keeping it open for respiration and
helps us vocalize.
The cricothyroid muscle lies anterior and external to
the larynx and was identified in the study of the muscular triangle of
the neck. It arises from the cricoid cartilage and attaches into the inferior
horn and lower margin of the thyroid cartilage. When it contracts, it pulls
the thyroid cartilage forward, increasing the distance between the thyroid
and arytenoid cartilages and tensing the vocal cord. It is supplied by
the external laryngeal branch of the superior laryngeal nerve (X).

By removing one lamina of the thyroid cartilage, you can see the lateral
cricoarytenoid muscle. As can be seen in the diagram, this muscle arises
from the upper border of the cricoid cartilage and inserts onto the muscular
process of the arytenoid cartilage.

The remainder of the major muscles can be seen from the back of the
larynx. Located in the upper part of the aryepiglottic fold is the aryepiglottic
muscle. It is attached to the lateral border of the epiglottis and becomes
the oblique arytenoid which then attaches into the arytenoid cartilage.
This muscle works as a purse string to close the opening of the larynx
when swallowing, protecting the larynx.
Running from arytenoid to arytenoid cartilages is the
transverse arytenoideus muscle. This muscle pulls the arytenoids together
when they contract.
Lastly, we have the posterior cricoarytenoid muscles.
They arise from the expanse of the cricoid lamina and insert into the muscular
process of the arytenoid cartilages. These are the only muscles that open
the space between the vocal cords (abduct).
Another muscle, not shown, is the thyroarytenoideus. This
muscle extends from the back of the thyroid cartilage to the front side
of the arytenoid cartilage. It pulls the arytenoid cartilage forward when
it contracts, thus loosening the vocal ligament.
The deep upper part of this muscle is the vocalis muscle. This muscle
can change the tenseness of small segments of the vocal cord in order to
vary tonal qualities of our voice.

Actions of Laryngeal Muscles

I consider that the major function of the larynx is to keep the airway
open. This means keeping the space between the vocal cords (rima glottidis)
open. If it isn't open, we don't breathe so the second function doesn't
matter.
The second important function is vocalization and this
is a very complicated procedure that requires a variety of parts of the
body to function together.
Figure 1 points out the parts of the larynx involved in breathing
and vocalization.

arytenoid cartilage

vocal process

muscular process

Figure 2 shows the movements that take place between the arytenoid and
cricoid cartilages (cricoarytenoid joints). The dot in the arytenoid cartilage
is the vertical axis around which the arytenoid cartilage rotates.
The movements include:

adduction (AD)

abduction (AB)

anterior-posterior sliding (AP)

medial-lateral sliding (ML)

Figure 3 shows the action of the transverse arytenoideus muscle. The arytenoid
cartilages are pulled towards each other, thus closing the rima glottidis.

Figure 4 shows that the lateral cricoarytenoid muscles adduct the arytenoid
cartilages to close the rima glottidis.

Figure 6 shows the only abductor, the posterior cricoarytenoideus muscle.
This muscle rotates the arytenoid cartilages laterally (abduct), causing
the vocal cords to separate from one another, opening the rima glottidis.

Figure 7 shows the cricothyroid muscle. As mentioned before, this muscle
is external to the larynx and can be seen in the muscular triangle of the
neck. This muscle rotates the thyroid cartilage forward around an axis
through the cricothyroid joint. This action stretches the vocal cord, thereby
tensing it.

Figure 1.
Figure 2.

Figure 3.
Figure 4.

Figure 5.
Figure 6

Figure 7.

Consideration of All of the Structures That Take Part In Vocalization

In considering the process of speech, you must first understand that
sounds are produced by blowing air past the vocal cords. The air produces
vibrations in the vocal cords, forming the sounds that come out of the
mouth. The air is pushed out of the lungs by relaxing the diaphragm and
contracting muscles of the abdominal wall. The abdominal muscles are supplied
by the lower intercostal nerves.

The tonal and pitch variations occur when the vocal cord is made more
tense or looser. Tense cords produce higher pitch, loose cords form lower
pitch. The muscles involved are supplied by the recurrent
laryngeal and external laryngeal nerves.

Also changing the vocal cords, we have the stylopharyngeus
muscles that pull the pharynx and larynx upward. This serves to shorten
the tubal length of the air passage between the base of the skull (body
of sphenoid) and the vocal cords. As found in an organ, longer pipes are
low pitch and shorter pipes are high pitch. The stylopharyngeus are supplied
by the glossopharyngeal nerves.

The suprahyoid muscles (digastrics, geniohyoid)
pull the hyoid bone up and the larynx follows it. The anterior digastric
is supplied by the nerve to mylohyoid.
The posterior belly of the digastric is supplied by the facial
nerve. The geniohyoid muscle is supplied by
C1.

The infrahyoid muscles (sternothyroid, omohyoid)
pull the larynx down. They are supplied by branches of the ansa cervicalis
(C1-C3).

The tongue is used to add various inflections
to our voice by varying its surface contour (intrinsic muscles) supplied
by the hypoglossal nerve.

Muscles of facial expression, especially around
the mouth and cheeks, are also used to vary the quality of our speech,
facial
nerve.

Finally, if you notice what must move when we
speak, the mandible moves up and down by contraction of the muscles of
mastication, mandibular division of trigeminal
nerve.

This description of speech production is an oversimplification,
but gives you an idea of just how many nerves and muscles are involved
in providing us with the ability to speak.

Another function of the muscles of the larynx is protection of the
airway. The major muscle that performs this protection is the aryepiglottic
muscle. When you swallow, the aryepiglottic muscle closes off the entrance
to the larynx through a purse-string action. It pulls the sides of the
epiglottis back and narrows the inlet to the larynx. Muscle responsible
is supplied by the recurrent laryngeal nerve.
To help in this protection, all muscles that adduct or tense the vocal
cord can also narrow the rima glottidis protecting the trachea and lungs.

The cough reflex is also designed to protect the air way. Should something
get into the vestibule of the larynx, sensory fibers from the internal
laryngeal nerve carries a signal to the central
nervous system and from there an impulse is sent to the abdominal muscles
which then force a blast of air through the larynx which expels the foreign
object.

Sagittal Section Through Larynx

The sagittal view of the larynx reveals relationships that aren't otherwise
seen.
The aryepiglottic folds form the borders of the opening
to the larynx. You can see that they extend from the epiglottis to the
arytenoid cartilages.
The first space in the larynx is called the vestibule
which extends down to the vestibular fold (false vocal cord).
Just beneath the vestibular fold is the ventricle which
extends laterally and is connected anteriorly to a small sac called the
saccule. The saccule has special secretory cells that produce a secretion
to keep the vocal cords moist.
Beneath the ventricle is the true vocal cord that extends from
the vocal process of the arytenoid cartilage to the back side of the thyroid
cartilage.
The thin space between the left and right vocal cords
is called the rima glottidis through which air must pass in order to vocalize
and breathe.
The area below rima glottidis is called the infraglottic
cavity.
Below the infraglottic cavity is the trachea.